The 5-4 ruling is a reversal of campaign-finance decisions the court issued in 1990 and 2003, and the chief difference between now and then is Justice Alito, who is sitting in the slot formerly held by retired Justice Sandra Day O'Connor.

“The replacement of O’Connor with Alito totally transformed this area,” said Richard Briffault, a professor at Columbia Law School.

He said that in the 10 years before Justice O’Connor’s retirement, there were four cases that upheld campaign-finance regulations, and in the years since her retirement in 2006, there have been four cases that either struck down or narrowed campaign regulations that the court “clearly would have upheld had O’Connor been on the court.”

The decision, which held that corporations and unions have the same ability to pay for political ads as an individual would, gave an inside look at how the makeup of a court can control the outcome of a case. In doing so, it underscored just how much rides on Supreme Court nominations.

Justice Alito isn’t the only new member of the court since 2003, when the court last upheld restrictions on independent political expenditures by corporations, unions and associations. Chief Justice John G. Roberts Jr. replaced deceased Chief Justice William H. Rehnquist in 2005, and Justice Sonia Sotomayor has replaced Justice David H. Souter, who retired last year.

But the other two were replacing justices with similar judicial views, while Justice Alito is markedly more conservative on the court than Justice O’Connor - particularly on campaign-finance cases.

Justice John Paul Stevens, writing in dissent, pointedly noted the difference between then and now.

“The only relevant thing that has changed [since Austin v. Michigan Chamber of Commerce and McConnell v. Federal Election Commission] is the composition of this court. Today’s ruling thus strikes at the vitals of stare decisis,” Justice Stevens wrote.

Couched in the niceties of legal language, he called the majority out, arguing that it had ignored the cardinal principle of judicial restraint that if a case can be decided narrowly, then it would be wrong to issue a broader ruling than necessary.

That sparked a retort from Chief Justice Roberts and Justice Alito, both of whom were quizzed heavily on their fealty to judicial restraint during their confirmation hearings.

“Sometimes it is necessary to decide more. There is a difference between judicial restraint and judicial abdication,” Chief Justice Roberts wrote in a concurring opinion joined by Justice Alito.

“So much for the Justice Roberts view of modesty, of following precedent,” said Sen. Charles E. Schumer, New York Democrat. He pronounced himself “appalled” at the chief justice and Justice Anthony M. Kennedy, who wrote the court’s majority opinion.

Hans A. von Spakovsky, a former member of the Federal Election Commission who is now senior legal fellow at the Heritage Foundation, said Justice Kennedy has now become the court’s center, taking over from Justice O’Connor.